Digital regulator and method of current regulation

ABSTRACT

A digital regulator for controlling the amount of current supplied to a load from parallel-connected rectifiers. If no rectifier is overloaded, the regulator maintains the magnitude of load current at a preselected level by either raising the output of the rectifier which is supplying the least current or by lowering the output of the rectifier which is supplying the most current. If, however, any rectifier becomes overloaded, the regulator switches to a mode of operation in which it allows the overloaded rectifier to operate in its overloaded state for a predetermined period of time, after which its current output is lowered. If the lowering of the current output of the overloaded rectifier causes the magnitude of load current to drop below the preselected level, the regulator will, after lowering the output of the overloaded rectifier, raise the output of the rectifier which has been continuously operating in range for the longest period of time.

United States Patent 1 DePuy [S4] DIGITAL REGULATOR AND METHOD OF CURRENT REGULATION [52] U.S. Cl .Q ..32l/14, 32l/25, 321/27 R [51] Int. Cl. ..H02m 1/18 [58] Field of Search ..32l/5, 14,25, 26, 27

[56] References Cited I UNITED STATES PATENTS [451 Apr. 3, 1973 Primary Examiner-William M. Shoop, Jr. Attrney.l. Wesley Haubner et al.

[57] ABSTRACT A digital regulator for controlling the amount of current supplied to a load from parallel-connected rectifiers. If no rectifier is overloaded, the regulator maintains the magnitude of load current at a preselected level by either raising the output of the rectifier which is supplying the least current or by lowering the output of the rectifier which is supplying the most current. If, however, any rectifier becomes overloaded, the regulator switches to a mode of operation in which it allows the overloaded rectifier to operate in its overloaded state for a predetermined period of time, after which its current output is 5,1941 Moyer et R lowered. If the lowering of the current output of the 2,845,591 7/1958 Albrecht et al ..321/27 2 891 212 6/1959 Bingham 32 X overloaded rectifier causes the magnitude of load cur- 3 172 093 3 9 Diebold x rent to drop below the preselected level, the regulator 3,205,423 9/1965 Kanngiesser... 321/27 R X will, after lowering the output of the overloaded recti- 3,241,052 3/1966 Day ..32l/25 X fier, raise the output of the rectifier which has been 3,270,271 8/1966 Stuefen ..321/25 continuously operating in range for the longest period 3,624,405 11/1971 Bishop et a1... ....32l/25 X f ti 7 3,662,250 /1972 Piccone et al. 32l/27 R 13 Claims, Drawing Figures T i g l I 675/ -00W/V 1/ l 1 mn/vsromm? RECT/F/ER MA 6/v7'/c ,R:cr/r/R 1 w/ m up TRANSFORMER AMPL/F/ER u/v/r I c/m/vsaw /c M I Z w I l 575/ -00WA/ 12 I z m 6/? MAGNET/C RECT/F/E/i vi/$ 731 r/ mis roirMtk AMPL/F/ER (IN/7 l f I CHANGER -31, -20 -2d f, :::::::::::1111 40/40 $2 2 u 1 '1 .s an E a c 7255755272,? near/mm MA G/VET/C fifCf/F/ER l I w/m mp mA/vsro/wm AMPL/F/fl um?" I I CHANG is as -33 "id J F W g 1| 1/ [2 [3 14 i STEP-Dow REC mun MAG/VT/C RECT/F/ER H l 6 i i i 7 m 7 21 TRAA/J/GRMIR AMPLIFIER u/v/r [a I CHM/R 4.: -41, 4d I h i E v w l 70 Ja PATENTEDAPRB I975 sum 02 0F 15 PATENTEUAFR3 ms sum 05 0F 15 gull.

PATENTEUAPR3 I973 3,725,769

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SHEET llUF 15 CLOCK A I 293 cLoc/r 5 CLOCK C 297 CLOCK 0 30/ DIGITAL REGULATOR AND METHOD OF CURRENT REGULATION BACKGROUND AND OBJECTS OF THE INVENTION This invention relates generally to regulation systems for current supplies and more particularly to automatic regulators for use in parallel-rectifier, high-current supply systems. The following art now known to applicant is indicative of prior art approaches relevant to this area of technology: U.S. Pat. Nos. 1,849,518 (Gay); 2,243,572 (Moyer et al); 2,845,591 (Albrecht et al); 3,229,] 87 (Jensen); and 3,270,271 (Stuefen).

In electrical systems requiring the production of high levels of direct current, e.g., the electrolysis of brine into chlorine, several rectifier groups including associated transformers may be connected in parallel with one another to supply current to the load. In order to ensure that the level of current which is supplied to the load is maintained at a preselected level, regulation equipment is commonly coupled to the rectifiers. Such equipment is operative for raising the output current of a rectifier if the load current is sensed as being below a preselected level and for lowering the output current of a rectifier if the load current is sensed as being above a preselected level.

Unless each of the rectifiers of the supply system have identically matched characteristics and are operated at the same output voltages, during parallel operation there will probably be differences in the amount of current that each provides to the load. In some situations the variation in output voltages among the rectifier groups can result in a rectifier group providing substantially more than full rated current (e.g., 110 percent).

Accordingly, when parallel rectifier current supply systems are designed it is a common practice to provide each rectifier with a, rectifier transformer having the capability of continuous operation at levels wherein its associated rectifier is supplying full rated current plus the increment of current which is attributible to the voltage difference among the rectifier groups in the system. Such transformers can be rather expensive.

It is the main object of my invention to provide a regulation system for a parallel rectifier, high current supply system which enables the utilization of smaller rectifier supply transformers than with prior art systems.

It is a further object of my invention to provide a digital regulation system for parallel-rectifier, high current supply systems.

It is yet a further object of my invention to provide an improved method of regulating parallel rectifier units supplying high current to a load.

It is yet a further object of my invention to provide a digital regulation system which has plural modes of operation, one mode being automatically utilized whenever at least one rectifier group is providing more than full rated current (i.e., is overloaded) to ensure that heating of the rectifier supply transformers is kept to a minimum.

SUMMARY OF THE INVENTION In a parallel-rectifier, high-current supply system I provide an automatic regulator which monitors and controls the magnitude of current supplied to a load.

If the magnitude of load current is above a preselected level and all units are operating at or below full rated current the regulator causes the rectifier which is supplying the most current to decrease its output. Conversely, if the magnitude of load current is below a preselected level the regulator causes the rectifier which is supplying the least current to increase its output.

If any rectifier becomes overloaded, i.e., provides more than full rated current, the regulator automatically switches to a mode of operation in which the overloaded rectifiers(s) is allowed to operate in the overloaded condition for a predetermined period of time. Upon the termination of that period of time the regulator automatically decreases the output of the rectifier which has been overloaded for the longest immediately preceding period of time. If, in response to the decreasing of the output of the overloaded rectifier, the load current drops below the preselected level, the regulator automatically raises the output of the rectifier which has been operating inrange (i.e., at or below full rated current) continuously for the longest immediately preceding period of time.

The time period during which a rectifier is allowed to operate in an overloaded condition is predetermined and is sufficiently short so as to minimize excess heating of the rectifiers supply transformer (i.e., the period is short compared to the thermal time constant of the rectifiers supply transformer).

When all of the systems rectifiers are again operating in range and the regulator determines that the load current is above the preselected level the regulator automatically switches back to the mode of operationwherein the maintenance of the preselected magnitude of load current is effectuated by either raising the output of the rectifier which is supplying the least current or by lowering the rectifier which is supplying the most current.

DESCRIPTION OF THE DRAWINGS This invention will be better understood and its various objects and advantages will be more fully appreciated from the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a functional block diagram of a plural rectifier current supply system including my digital regulator;

FIG. 2 is a functional block diagram of my digital regulator;

FIG. 3 is a schematic diagram of the Reference and Main Comparator 7 shown in FIG. 2;

FIG. 4 is a schematic diagram of the Raise-Lower Decoder 8 shown in FIG. 2;

FIG. 5 is a schematic diagram of the Up-Down Counter 9 shown in FIG. 2;

FIG. 6 is a schematic diagram of the Raise-Lower Pulse Generator 10 shown in FIG. 2;

FIG. 7 is a schematic diagram of the Unit Feedback Amplifier 12 shown in FIG. 2;

FIG. 8 is a schematic diagram of the Unit Comparator 13 shown in FIG. 2;

' FIG. 9 is a schematic diagram of the High-Low Hold Circuit 14 shown in FIG. 2;

FIG. 10 is a schematic diagram of the High-Low Decoder 15 shown in FIG. 2;

FIG. 11 is a schematic diagram of the High-Low Switch 16 shown in FIG. 2;

FIG. 12 is a schematic diagram of the Unit Overload Comparator 19 shown in FIG. 2;

FIG. 13 is a schematic diagram of the Overload Decoder 20 shown in FIG. 2;

FIG. 14 is a schematic diagram of the Overload Sequence Control and Position Used Circuit 21 shown in FIG. 2;

FIG. 15 is a schematic diagram of the Overload Sequence Control and Position Used Circuit 22 shown in FIG. 2;

FIG. 16 is a schematic diagram of the Overload Aux- I iliary Control Circuit 23 shown in FIG. 2;

FIG. 17 is a schematic diagram of the Timer 24 shown in FIG. 2; I

FIG. 18 is a schematic diagram of the Raise-Lower Activate Circuit shown in FIG. 2;

FIG. 19 is a schematic diagram of the Magnetic Amplifier Activate and Relay Circuit 18 shown in FIG. 2; and

FIG. 20 is a schematic diagram of the Tap Changer Activate and Relay Circuit 17 shown in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is a functional block diagram of a system for .supplying high levels of current to a load. As can be seen four rectifier groups, i.e. 1, 2, 3 and 4, are connected in parallel with one another between a three phase source of alternating voltage and a direct current load 5.

Each rectifier group is adapted for stepping down the voltage from the three phase source and converting it to a unidirectional or D-C current and for supplying that current to the load 5. To that end each rectifier group includes a step-down transformer having associated therewith a tap changer, a rectifier transformer, a magnetic amplifier and a rectifier unit. For

- example, group 1 includes step-down transformer la,

rectifier transformer 1b, magnetic amplifier 1c, and rectifier unit Id. In a similar manner group 2 includes step-down transformer 2a, rectifier transformer 2b, magnetic amplifier 2c and rectifier unit 2d. Group 3 includes step-down transformer 3a, rectifier transformer 3b, magnetic amplifier 30 and rectifier unit 3d, and group 4 includes step-down transformer 4a, rectifier transformer 4b, magnetic amplifier 4c and rectifier unit 4d.

The magnitude of the portion of total load current that each rectifier unit supplies is a function of the voltage applied thereto..Accordingly, a change in the magnitude of the output current which is provided by any individual unit can be effectuated by either raising or lowering the voltage applied to that unit.

I The raising or the lowering of the voltage applied to a rectifier unit is accomplished digitally by changing the state of the magnetic amplifier and, if needed, by altering the tap setting of the step-down transformer tap changer. To that end the tap changer includes sufficient settings or steps to cover the voltage range necessary to regulate a units output current, with each step effecting a predetermined incremental (or decremental) change in the average magnitude of the rectifier voltage. The magnetic amplifier is so constructed and arranged that in one state (referred to hereinafter as saturated) it has negligible effect, while in its other state (unsaturated) it stands off enough voltage to cause a reduction in the average magnitude of the rectifier voltage approximately equal to one-half of the aforesaid predetermined change. The half step thus afforded by the magnetic amplifier provides at relatively low cost the resolution necessary for current regulation accuracy.

The manner in which the tap changer and the magnetic amplifier control the voltage on their associated rectifier unit can be best understood from the following example. In order to raise the voltage applied to a rectifier unit the equivalent of one half of a tap setting (which is the smallest incremental voltage change possible), if the magnetic amplifier is unsaturated the change is effectuated by merely saturating the magnetic amplifier while leaving the tap on the step-down transformer tap changer at its existing setting. The saturation of the magnetic amplifier will result in raising the average voltage of the rectifier a half step to the level determined by that particular tap setting.

If the magnetic amplifier is saturated, a one-half step raise is effectuated by desaturating the amplifier and at the same time raising the tap setting on the tap changer by one full step. The net effect of the one step rise in voltage (resulting from the changed tap setting) and the half step drop in voltage (resulting from the desaturation of the magnetic amplifier) results in a one-half step rise in the voltage applied to the rectifier unit.

As should be apparent, the voltage applied to a rectifier unit can be lowered by one half of a step by the opposite procedure which is utilized to effect the one half step increase.

As can be seen in FIG. 1 the unidirectional output current of unit 1 is denoted as I1, the unidirectional output current of unit 2 is denoted as I2, the unidirectional output current of unit 3 is denoted as I3 and the unidirectional output current of unit 4 is denoted as I4. The sum of currents I1, I2, I3 and I4 is denoted as I5 and is delivered to the load 5.

It should be pointed out at this juncture that each of the rectifier units may include one or a plurality of parallel connected rectifiers depending upon the magnitude of output current desired therefrom.

The alteration of the tap changers position and the magnetic amplifiers state is accomplished automatically by the digital regulator of my invention. In so doing the regulator is able to regulate the amount of current supplied to the load to either maintain it at a preselected level or to raise or lower it to another preselected level.

The digital regulator of my invention is shown as a functional block 6 in FIG. 1.

My regulator receives its inputs I1, I2, I3 and I4 which are currents proportional to the group output currents I1, I2, I3 and I4, respectively. The output of the digital regulator consists of signals which are utilized to change the tap changer setting for each groups step-down transformer tap changer and to change the state of each groups magnetic amplifier.

The regulator has two modes of automatic operation, one mode is what I choose to call current basis operation and the other mode is what I choose to call sequence basis operation.

rent or is overloading its associated rectifier transformer).

Current basis operation entails determining if the magnitude of load current is at a preselected level and if not, for altering the voltage on selected rectifiers to bring the load current to that level. To that end, if the regulator 6 determines that the load current is below the preselected level, the voltage on the rectifier unit which is supplying the least current will be raised automatically in order to increase its output current and to bring the load current up to the desired level. If the regulator determines that the load current is above the preselected level, the voltage on the rectifier unit which is supplying the most current will be lowered automatically in order to decrease its output current and bring the load current down to the desired level.

The regulator operates on a sequence basis whenever any rectifier group is providing more than full rated current. Sequence basis operation also entails maintaining the load current at a preselected level although the manner which such regulation is accomplished differs from current basis operation. To that end the regulator 6 determines the order in which the rectifier units began and have continuously remained in range (i.e., it determines which unit has been continuously in range the longest period of time, which unit has been continuously in range the next longest period of time, and so on) and also determines the order in which the rectifier units began and have continuously remained in an overloaded condition (i.e., it determines which unit has been continuously overloaded the longest period of time, which unit, if any, I

has been overloaded the next longest period of time, and so on).

After a preselected period of time the voltage on the rectifier unit which has been overloaded the longest period of time is decreased, thereby allowing its associated rectifier transformer to cool down. If the controller determines that the magnitude of load current flowing is below the preselected level (due to the lowering of the voltage on the overloaded unit) it automatically raises the voltage on the unit which has been in range continuously for the longest immediately preceding period of time. After another similar length period of time the voltage on the overloaded rectifier unit is decreased, thereby allowing its associated transformer to cool down. If the regulator then determines that the magnitude of load current is below the predetermined level (e.g., due to the lowering of the voltage on the overloaded unit) it automatically raises the voltage on the unit which has, at that time, been in range continuously for the longest immediately preceding period of time.

Sequence basis operation continues so long as any unit remains overloaded. As soon as conditions are such that no unit is overloaded but the magnitude of load current is above the preselected level the regulator automatically switches back to current basis operation.

The portion of the digital regulator which effectuates current basis operation can be seen in the functional block diagram of FIG. 2 and includes a Reference and Main Comparator Circuit 7, a Raise-Lower Decoder Circuit 8, an Up-Down Counter Circuit 9, a Raise- Lower Pulse Circuit 10, a Raise-Lower Activate Circuit 11, a Unit Feed-back Amplifier 12, a Unit Comparator Circuit 13, a High-Low Hold Circuit 14, a High-Low Decoder Circuit 15, a High-Low Switch 16, a Tap Changer Activate and Relay Circuit 17 and a Magnetic Amplifier Activate and Relay Circuit 18. Each of these circuits is provided with suitable clock pulses from pulse generators, not shown, to ensure that proper operation of the regulator ensues.

The reference in main comparator circuit 7 sums the are proportional to currents l1, 12, I3 and I4, respectively and are provided by current transducing means, not shown. The sum of signals l1, l2, l3 and I4 is denoted as 15' and is proportional to the magnitude of load current [5. The sum current signal I5 is compared in the comparator 7 to a preestablished dead-band reference voltage (i.e., a voltage band bounded by an upper limit voltage and by a lower limit voltage). The upper limit of the comparators dead-band reference voltage is adjustable and defines the maximum level of load current which the regulator will allow to flow before it lowers the voltage on any rectifier unit. The lower limit of the dead-band voltage is also adjustable and defines the minimum level of load current that the regulator 'will allow to flow before it raises the voltage on any rectifier unit.

If the sum signal I5 is lower than the lower deadband limit, a raise" signal is provided by the circuit 7. This raise signal is provided via two conductors and consists of a binary O on each conductor. If the sum signal is higher than the upper dead-band limit a lower signal is provided by the circuit 7. The lower signal consists of a binary l on each conductor. If the sum signal is within the dead-band (indicating that the magnitude of load current is at the preselected level) the output of the comparator circuit consists of a binary 1 signal on one output wire and a binary 0" signal on the other output wire. These signals indicate that no change is required.

The output signals from the comparator 7 are decoded by the Raise-Lower Decoder 8.

The output of the Raise-Lower Decoder 8 is provided via two wires, one wire being a raise" wire and the other wire being a lower wire. If the comparator output is a O and 0 the decoder 8 provides a binary l signal at its raise output, if the comparator output is a l and 1 the decoder provides a binary l signal at its lower output. If the output of the comparator is a 1 and 0 the decoder doesnt provide a binary l at either of its outputs. The signal from the decoder is therefore either a raise or a lower" or a no change signal and is supplied as an input to the Up-Down Counter Circuit 9.

The Up-Down Counter Circuit is provided to delay a change in the voltage on any rectifier unit, if such a change is desired, until any transients which may have arisen in the system will have subsided. To that end the counter is designed to count the number of clock pulses provided to it, up to a prescribed time-out point in response to the continuance of a raise signal from the decoder 8 and to count the number of clock pulses provided to it, down to a prescribed time-out point in response to the continuance of a lower" signal therefrom. If in counting up to the upper time-outpoint the raise signal from the decoder terminates before the counter reaches that point and is replaced by a lower signal, the counter will begin counting down from the point it reached in counting up.

Upon reaching the upper time-out point (in response to a sustained raise signal) the counter provides a raise enable signal at one of its output terminals. Upon reaching the lower time-out point (in response to a sustained lower" signal) the counter provides a lower enable signal at another of its output terminals. Any time the counter reaches a time-out point its clocking pulses are suppressed.

The raise enable or lower enable signals are provided as inputs to the Raise-Lower Pulse Generator 10. This circuit is operative for generating a raise pulse in response to the raise enable signal or a lower pulse in response to the lower enable signal. To that end the generator includes a clocked counter which terminates the raise or lower enable signal after a preselected short period of time to produce a raise or lower pulse. The counter also provides a signal to reset the Up-Down Counter Circuit 9 upon reaching a preselected point in its count.

The raise or lower pulse from the generator 10 is provided as one input to the Raise-Lower Activate Circuit 1 1. This input carries the information as to whether or not the voltage on any rectifier unit needs to be raised or lowered to achieve the desired level of load current.

The Raise-Lower Activate Circuit 11 also receives the second input, which carries information as to which rectifier unit is low (i.e., needs to be raised next in the event that a raise is required), and which unit is high (i.e., needs to be lowered next in the event that a lower is required). Such information is provided in the following manner. The unit feedback amplifier 12 takes the four currents I1, 12', I3 and I4 and converts each into a pair of complementary voltages proportional to the magnitude of current provided by its associated rectifier unit. For example, current II, which is proportional to the current output of rectifier unit 1d, is converted into complementary voltages +V1 and VI, each of which is proportional to the magnitude of the output current of the rectifier'unit 1d. Similarly provided complementary voltages +V2 and V2 are each proportional to the output of rectifier unit 2d, complementary voltages +V3 and --V3 are each proportional to the output of rectifier unit 3d and complementary voltages +V4 and V4 are each proportional to the output of rectifier unit 40'.

The complementary voltages are provided as inputs to the Unit Comparator Circuit 13. This circuit includes six subcircuits each of which compares the positive complementary voltage of one unit to the negative complementary voltage of another unit in order to determine which is of greater absolute magnitude. The output of all rectifier units are thus compared to one another in the unit comparator and signals indicating the relative level of the rectifier units output currents are provided at its output, i.e., the comparator provides six output signals, one comparing the current outputs of units 1d and 2d, another comparing the outputs of units 1d and 3d, a third comparing the outputs of units 1d and 4d, a fourth comparing the outputs of units 2d vided as an input to a High-Low Decoder Circuit 15 which decodes the comparison information.

The output of decoder 15 contains binary signals indicating which unit is providing the most current to the load and which unit is providing the least current thereto. I

The decoders binary output signals are provided as one input, called the current basis input, to the High- Low Switch 16. This switch includes circuitry operative for enabling the binary output signals from the decoder 15 to pass to the second input of the Raise-Lower Activate Circuit 11 if the regulator 6 is operating on a current basis. I

If any unit becomes overloaded, such that the regulator begins operating on a sequence basis, a control signal indicating this fact is provided to the switch 16 to disable it from passing the decoder 15 binary signals to the activate circuit 11 and to enable it to pass sequence basis binary signals to the second input of the activate circuit.

Assuming that nounits are overloaded and that the Pulse Generator Circuit 10 is providing either a raise" or lower pulse, the binary output signals of the decoder 15 and the raise or lower pulse are combined in the Raise-Lower Activate, Circuit 1 1. v

A raise pulse coupled with a signal indicating that a particular unit is providing the least current to the load is combinedin activate circuit 11 to provide an output signal indicating that such a unit is to have its voltage raised. Similarly, a lower" pulse coupled with a signal indicating that a particular unit is providing the most current is combined in the activator circuit to provide an output signal indicating that such a unit is to have its voltage lowered.

The output signals of the Raise-Lower Activate Circuit 11 are provided as inputs to the Magnetic Amplifier Activate and Relay Circuit 18 and to the Tap Changer Activate and Relay Circuit 17.

Any time that a voltage change is to be made i.e., any time that raise-lower activate circuit 11 provides output signals) the magnetic amplifier activate 18 is operated. This action results in either saturating or desaturating the magnetic amplifier for the unit to be changed depending on its immediately past state. The tap changer activate 17 is operated in response to the output of the Raise-Lower Activate Circuit 11 as well as the Magnetic Amplifier Activate and Relay Circuit 18. To that end assuming that unit 1d is to be raised and that its magnetic amplifier 1c is in its saturated state, upon the coincidence of a raise pulse and a binary signal indicating that unit 1d is low, the Raise-Lower- Activate circuit provides an output signal to both the Magnetic Amplifier Activate and the Tap Changer Activate circuits. This output signal effectuates the 

1. In an electrical system comprising a plurality of parallel connected rectifier units for supplying current to a load, each of said units being connected to a separate rectifier transformer to supply a portion of the total load current the magnitude of which portion is a function of the voltage applied to that unit, each of said units being operative ''''in range'''' when supplying no more than a predetermined rated magnitude of current and being capable of ''''overloaded'''' operation in which condition more than said predetermined rated magnitude of current is supplied, the system further comprising voltage adjusting means for individually adjusting the voltage applied to each of said rectifier units and a regulator including means for monitoring the magnitude of current supplied to the load and means operative when all units are operating ''''in range'''' for causing said voltage adjusting means to raise the voltage on the rectifier unit which is providing the smallest fraction of its rated current if the magnitude of load current is below a preselected minimum level and for causing said voltage adjusting means to lower the voltage on the rectifier unit which is providing the largest fraction of its rated current if the magnitude of load current is above a preselected maximum level, improved means for minimizing overheating of any rectifier transformer in the event that its associated unit is operating ''''overloaded,'''' said means comprising: a. first means for individually detecting ''''overloaded'''' operation of the respective rectifier units; b. second means responsive to said first means detecting an ''''overloaded'''' operation for determining the sequential order that said units began ''''overloaded'''' operation and for providing a first signal indicating which unit has been ''''overloaded'''' for the longest immediately preceding period of time; and c. third means responsive to said first signal and operative whenever a detected ''''overloaded'''' condition has continuously existed throughout a predetermined period of time for causing said voltage adjusting means to lower the voltage on whichever rectifier unit said first signal indicates has been ''''overloaded'''' for the longest immediately preceding period of time.
 2. The electrical system as specified in claim 1 additionally comprising: d. fourth means, coupled to said first means, for determining the sequential order that said units began ''''in range'''' operation and for providing a second signal indicating which unit has been ''''in range'''' for the longest immediately preceding period of time; and e. fifth means responsive to said third signal and operative for causing said voltage adjusting means to raise the voltage on the unit which said second signal indicates has been operating ''''in range'''' for the longest immediately preceding period of time if the level of load current drops below the preselected minimum level after the operation of said third means.
 3. The electrical system as specified in claim 1 wherein said first means comprises: i. fourth means for providing plural signals proportional to the magnitude of current supplied to the load; and ii. fifth means coupled to said fourth means for producing a set of second signals indicating which, if any, of said plural signals is greater than a predetermined value, said second signals being supplied to said second means which is operative to determine the sequential order that said second signals were produced.
 4. The electrical system as specified in claim 3 wherein said second means comprises: storage means for receiving said second signals in sequential order and for providing said first signal indicating which unit has been continuously ''''overloaded'''' for the longest immediately preceding period of time.
 5. The electrical system as specified in claim 4 wherein said fifth means also provides a third signal indicative of any of said plural signals being greater than said predetermined value, and wherein said third means comprises: i. timer means adapted for producing a fourth signal in delayed response to said third signal; and ii. sixth means responsive to said first and fourth signal for causing said voltage adjusting means to lower the voltage on whichever unit said first signal indicates has been ''''overloaded'''' for the longest immediately preceding period of time.
 6. The electrical system as specified in claim 5 additionally comprising: d. seventh means, coupled to said first means, for determining the sequential order that said units begin ''''in range'''' operation and for providing a fifth signal indicating which unit has been ''''in range'''' for the longest immediately preceding period of time; and e. eighth means responsive to said fifth signal for causing said voltage adjusting means to raise the voltage on the unit which said fifth signal indicates has been operating ''''in range'''' for the longest immediately preceding period of time if the level of load current drops below the preselected minimum level after operation of said third means.
 7. The electrical system as specified in claim 6 wherein said fifth means also provides a set of sixth signals indicating which of said plural signals is equal to or less than said predetermined value, and wherein said seventh means comprises storage means adapted for receiving said sixth signals in sequential order and for providing said fifth signal indicating which unit has been operating ''''in range'''' for the longest immediately preceding period of time.
 8. The electrical system as specified in claim 7 wherein said eighth means comprises: i. nintH means operative for monitoring the magnitude of load current and for providing a seventh signal when the magnitude of load current is below said preselected minimum; and ii. tenth means responsive to said fifth and said seventh signals for causing said voltage adjusting means to raise the voltage on whichever unit said fifth signal indicates has been operating ''''in range'''' for the longest immediately preceding period of time.
 9. The electrical system as specified in claim 8 wherein said voltage adjusting means comprises an individual tap changer connected to each rectifier transformer and an individual magnetic amplifier connected to each rectifier unit.
 10. A method for preventing damaging overheating of any one of a plurality of electric transformers which are respectively associated with a plurality of parallelconnected rectifier units supplying high-current to a load, wherein the magnitude of the portion of load current supplied by each rectifier unit is a function of the voltage applied thereto by its associated transformer, said transformer being overloaded when its associated rectifier unit is providing more than a predetermined rated current to the load, comprising the steps of: a. separately monitoring the current provided by each rectifier unit to the load so as to indicate overload conditions; b. keeping track of the history of the overload conditions among all of the rectifier units and indicating which rectifier unit has been providing more than the predetermined rated current for the longest immediately preceding period of time; and c. automatically lowering the voltage on the rectifier unit indicated in step (b) whenever an overload condition has persisted for a predetermined period of time.
 11. The method as specified in claim 1 additionally comprising the steps of: d. indicating which rectifier unit has been providing equal or less than said predetermined rated current for the longest immediately preceding period of time; e. determining if the magnitude of total load current is below a preselected minimum level; f. automatically raising the voltage on the rectifier unit indicated in step (d) after step (c) is effectuated if the magnitude of load current is below said preselected minimum level.
 12. A method of regulating the magnitude of current supplied to a load from a plurality of parallel-connected, high-current rectifier units, each of said units being connected to a separate electric transformer to supply a portion of the total load current the magnitude of which portion is a function of the voltage applied to that unit, each of said units being operative ''''in range'''' when supplying no more than a predetermined rated current and being capable of ''''overloaded'''' operation in which condition more than rated current is supplied, comprising the steps of: a. determining if the magnitude of load current is either above a preselected maximum level or below a preselected minimum level; b. determining, individually for each rectifier unit, if the unit is operating either ''''in range'''' or ''''overloaded''''; and c. so long as all of said units are operating ''''in range'''': i. raising the voltage on whichever unit is supplying the smallest fraction of its rated current if the magnitude of load current decreases below said preselected minimum level, or ii. lowering the voltage on whichever unit is supplying the largest fraction of its rated current if the magnitude of load current increases above said preselected maximum level; or d. alternatively, when any unit is operating ''''overloaded'''', performing the following steps: i. indicating which unit has been operating ''''overloaded'''' for the longest immediately preceding period of time, and ii. automatically lowering the voltage on that unit whenever an overloaded operation has persisted for a predetermined period of time.
 13. The method as specified in claim 12 including the following additionaL steps: iii. indicating the particular unit that has been operating ''''in range''''for the longest immediately preceding period of time, and iv. automatically raising the voltage on that particular unit if, after performing the step described in subparagraph (d) (ii), the magnitude of load current drops below said preselected minimum level. 